The invention concerns a diffusion brazing process for pieces of superalloys based on nickel or cobalt, i.e., refractory alloys having a content by weight of at least 50% nickel or cobalt.
It is to be noted that diffusion brazing is defined as a joining process of the type consisting of placing between the faces of metal pieces to be assembled (of similar composition, a metal joining layer of a similar overall composition but containing further at least one addition element (sometimes called "flux") which renders it more readily meltable than the alloy or alloys of the pieces and which is capable of diffusing in said pieces, so that when an appropriate heat treatment is applied to the assembly of said pieces and the said joining layer, the layer is melted at least partially and then resolidified by the diffusion of the addition element.
It is known to join pieces of superalloys by diffusion brazing, particularly castings. French Pat. No. 2,132,050 describes, for example, the application of a process of this type to the joining of nickel base superalloys. The element to be added of the joining layer must be chosen so that it does not produce embrittling phases during its diffusion into the superalloys. For this reason, elements such as aluminum, titanium and carbon must be eliminated in principle from said joining layer. The latter contains, for example, in percents by weight:
15 Cr; 15 Co; 3 B, rest Ni.
The execution of such a process requires great caution. The joining layer must be as thin as possible, because the thicker it is:
the higher the risk of porosity and the loss of metal, barring the placing of gaskets around the joint and of metal reserves, thereby creating a veritable mold;
the longer the diffusion;
the more the composition and the properties of the pieces to be joined are altered in the vicinity of the joining faces.
However, the thinner the joining layer, the higher the risk of the appearance of joint defects (voids, adhesions, etc.), which imposes the requirement of precision machining the joining surfaces of the pieces and to subject them to joining pressure during the execution of the process.
In actual fact, all of these difficulties are the result of the condition wherein the joining layer melts integrally. More precisely, its liquidus temperature is lower than the solidus temperature of the superalloys of the workpieces. If a joint of excellent quality is desired (to form, for example, a piece of complex configuration by the joining of elementary pieces), the conditions to be observed render it on occasion impossible to find on the market an alloy (in the form of a powder or foil, etc.), the composition of which corresponds effectively to that of the joining layer that must be used.